1. Field of the Invention
The invention relates to an image pickup apparatus and, more particularly, to an image pickup apparatus which is advantageously applied to a video camera using an image pickup device such as CCD or the like.
2. Related Background Art
FIG. 4 shows a functional block diagram of a conventional image pickup apparatus. In FIG. 4, an image formed on a CCD 2 by a lens 1 is converted into electric signals of R (red), G (green), and B (blue). After that, the electric signals pass through a sample and hold (S/H) and automatic gain control (AGC) circuit 5 and gains of the respective colors of R, G, and B are adjusted by a white balance (WB) adjusting circuit 6. Gamma characteristics of those signals are corrected by a gamma converting circuit 8. The gamma correction in this instance is generally set to .gamma.= about 0.45. The gamma corrected RGB signals are converted into the luminance signal and color difference signals by a matrix circuit 9 and, after that, a composite video signal is output from an encoder 13.
On the other hand, an image pickup apparatus for extracting the luminance signal and chrominance signals from different image pickup devices and synthesizing them and obtaining a video signal is known as a double-plate type image pickup apparatus.
FIG. 7 is a block diagram showing a construction of a conventional double-plate type image pickup apparatus. In FIG. 7, reference numeral 114 denotes a CCD for the chrominance signals; 115 a CCD for the luminance signal; and 116 a half mirror to divide the light to the CCD for the chrominance signals and the CCD for the luminance signal. The CCDs 115 and 114 are driven by a CCD driver 104 which is made operative by a clock signal from a clock generator 103. The light which passed through an image pickup lens 101 is separated by the half mirror 116 to the signal which goes to the CCD 114 for the chrominance signals and the signal which goes to the CCD 115 for the luminance signal and is formed as images on the CCD, respectively. The signals of R, G, and B are output from the CCD 114 and pass through an S/H and AGC circuit 105. After that, levels of those signals are adjusted by a WB adjusting circuit 106. The RGB signals supplied from the WB adjusting circuit 106 are gamma corrected by a gamma converting circuit 108 and, after that, they are supplied to a matrix circuit 109. The matrix circuit 109 forms color difference signals from the input RGB signals. The color difference signals are band limited by low pass filters 110 and 111 and are supplied to an encoder 113. An output from the CCD 115 for the luminance signal passes through the S/H and AGC circuit 105 and is subsequently gamma corrected by the gamma converting circuit 108. After that the signal is band limited by a low pass filter 112 and is input to the encoder 113. The encoder 113 converts the chrominance signals and luminance signal which were input into the composite video signal and outputs.
The white balance adjusting circuit 106 in the image pickup apparatus using color filters of the pure color type uses a system such that the R, G, and B signals are extracted and the amplitude of the G signal is set to a reference and amplification gains of the R and B signals are controlled. In case of applying such a system to the double-plate type camera, as a first method, there is a method whereby the processes are separated to the CCD 115 for the luminance signal and the CCD 114 for the chrominance signals and the color filters of RGB are attached to the CCD 114 for the chrominance signals and the white balance is obtained by only the output of the CCD 114 for the chrominance signals. As a second method, there is a method whereby the processes are separated to the CCD for the G signal and the CCD for the R and B signals and the white balance is obtained by the outputs of both of the CCDs.
In the above conventional apparatus, since the gamma correction is also executed to noises which are generated on the sensor and on its output stage, there is a problem such that the noises in the portion in which the signal level is relatively low are emphasized and an S/N ratio is deteriorated.
In the above conventional example, since the incident light is divided to the CCD for the chrominance signals and the CCD for the luminance signal, the incident light amount to each CCD decreases, so that there is a problem such that the S/N ratio of the luminance signal is deteriorated.
With respect to the white balance as well, in case of the first method, there is a problem such that since the signal of each of R, G, and B is extracted from the CCD for the chrominance signals, a sampling frequency with respect to one color is low, so that a color moire is likely to occur. It is, therefore, necessary to sufficiently drop the band by an optical low pass filter.
In case of the second method, although it is advantageous for the color moire, since the signals of two CCDs are used to form the luminance signal, there is a problem such that a positioning precision of the CCDs is severe.